1. Field of the Invention
The present invention relates to an exhaust apparatus connected to a V-type internal combustion engine that is mounted in an engine compartment which is located in a front part of a vehicle.
2. Description of the Prior Art
It is well known that a V-type internal combustion engine may be mounted in an engine compartment in a front part of a vehicle transversely to the longitudinal direction of the vehicle. When a catalytic converter is provided in an exhaust system for such a V-type engine, a pair of exhaust manifolds (that is, a front exhaust manifold and a rear exhaust manifold) are generally connected to the engine, an exhaust pipe having a junction is connected to the exhaust manifolds, and the catalytic converter is interposed in the exhaust pipe at a position downstream of the junction. Since the exhaust pipe extends toward a rear part of the vehicle, a front exhaust path extending from the front exhaust manifold to the junction of the exhaust pipe usually is made longer than a rear exhaust path extending from the rear exhaust manifold to the junction. The material and wall thickness of the pair of exhaust manifolds are usually set identically.
However, in such a conventional structure, since the front exhaust path is longer than the rear exhaust path, the quantity of heat radiated from the exhaust gas flowing through the front exhaust path becomes larger than that radiated from the exhaust gas flowing through the rear exhaust path, and by this large heat radiation from the front exhaust path, the temperature of the exhaust gas reaching the catalytic converter becomes relatively low. Moreover, since the wind due to running of the vehicle influences the front exhaust path more strongly than the rear exhaust path, the heat losses from the front exhaust path will be larger than those from the rear exhaust path, thereby dropping the temperature of the exhaust gas reaching the catalytic converter as a whole. When the temperature of the exhaust gas is low, it takes longer for the catalyst to warm up and be effective. Generally, a catalyst can purify exhaust gas only when the temperature of the catalyst is above a certain point. For that reason, it is desirable for the catalyst to be heated as quickly as possible so as to minimize emissions of noxious gases into the atmosphere.
With respect to the above problem, JP-A-60-17220 discloses a device that supplies secondary air into the front exhaust path, whereby unburnt gas and other combustibles are caused to oxidize, thereby raising the temperature of the exhaust gas in the front exhaust path.
However, since a complicated mechanism and control system for the introduction of secondary air are required in this structure, the exhaust system becomes complicated and expensive.